1. Field of the Invention
The present invention relates to a hydraulic braking system for use in an automotive vehicle, and more particularly to a hydraulic braking system which has a hydraulic booster for actuating a master cylinder in response to operation of a manually-operated member with a hydraulic power pressure supplied from a power source.
2. Description of the Prior Art
In conventional service braking systems for an automotive vehicle, there are provided a plurality of hydraulic circuits connecting wheel brake cylinders mounted on road wheels with a hydraulic braking pressure generator such as a master cylinder, so that when one of the hydraulic circuits is broken, braking operation is performed by the rest of the hydraulic circuits. In general, a tandem master cylinder is used in a conventional dual circuits system.
In order to reduce a force required to operate a manually-operated member, for example a depressing force applied on a brake pedal in braking operation, the hydraulic braking system is provided with a servo unit which is referred as a servo or a booster and which utilizes compressed air, intake manifold vacuum (for a vacuum booster), or hydraulic pressure (for a hydraulic booster) as a power source. The hydraulic booster is a booster which actuates the hydraulic braking pressure generator such as the master cylinder by the hydraulic power pressure supplied from the power source in response to depression of the brake pedal.
It has been proposed to employ the hydraulic booster, in the hydraulic braking system, as a dynamic hydraulic braking pressure generator in addition to the master cylinder. In other words, a hydraulic pressure generated by the hydraulic booster in response to depression of the brake pedal is applied directly to one of the hydraulic circuits. For example, as shown in Japanese Patent Laid-open Publication No. 59-227552, the hydraulic pressure generated by the hydraulic booster is applied to rear wheel brake cylinders in a front-rear dual circuits system in order to reduce the stroke of the brake pedal.
In Japanese Patent Laid-open Publication Nos. 62-244750 and 62-244751, there is disclosed a hydraulic booster of so-called stroke jumping type in which a power piston is divided into two pieces, one of which is moved relative to the other immediately after depression of the brake pedal to compensate for an initial idle stroke of the brake pedal. Therefore, the stroke of the brake pedal or the manually-operated member may be reduced.
However, in the above-described hydraulic braking systems, the characteristic of the hydraulic braking pressure in the wheel brake cylinder increasing in response to increase of the depressing force applied on the brake pedal, indicates a linear characteristic, so that a large depressing force must be applied on the brake pedal when a large braking force is needed. While it is preferable that the characteristic of the hydraulic braking pressure increasing in response to increase of the depressing force applied on the brake pedal is variable to meet a driver's choice or various driving characteristics, the characteristic in the above-described prior hydraulic braking system having the hydraulic booster can not be made variable.